Currently, third-generation mobile communication services by means of the Code Division Multiple Access (CDMA) scheme have just introduced in wireless (mobile) communication systems, including wireless terminals (user terminals), such as cellular phones. Also, next-generation mobile communication technologies capable of providing even faster communications have been under development. The 3rd Generation Partnership Project (3GPP) is studying the Long Term Evolution (LTE), as one of such next-generation mobile communication technologies.
In a mobile communication system, as a preparation to a wireless base station (evolved Node B: eNB) and a user terminal (user equipment: UE) for initiating a communication with each other, a channel is provided for the UE to initiate a transmission to the eNB. The 3GPP refers this channel as a “random access channel (RACH)”, and refers the procedure for initiating a communication through an RACH as a “random access (RA)”.
An RACH contains minimal information, using which the eNB distinguishes UEs from each other which make transmissions. The RACH is used when initiating a communication, and individual channels (or shared channels) are used afterwards. Thus, the RACH can be shared among a plurality of UEs unless it is used by more than one UEs simultaneously. For this purpose, identifiers called “signatures” (preambles) “are provided to the RACH, such that the eNB can distinguish UEs from each other which make transmissions simultaneously on the RACH.
A UE make an RA, for example, upon an initial transmission (origination), in response to an incoming transmission from the eNB (generation of downlink (DL) data), upon handover, upon recovery from disconnection (resuming a disconnected communication). As used herein, a wireless link in the direction from the eNB to a UE is referred to as “downlink (DL)”, and a wireless link in the opposite direction as “uplink (UL)”.
There may be some cases wherein an eNB does not recognize some UEs upon an initial transmission or recovery from disconnection, for example, and no individual signatures that can be exclusively used are allocated to the UEs. Such UEs randomly select one of a plurality of (for example, 64) preset signatures for making an RA. Accordingly, a plurality of UEs may simultaneously make RAs using the same signature, although the possibility of such an event is low. Such an RA procedure is referred to as a “contention based random access procedure (contention based RA procedure).
In such a case, the eNB resolves the conflicted signature (select one of the competing UEs), and sends a response to the selected UE. Each UE determines that that UE is selected by the eNB whether a response is received from the eNB. The UE selected by the eNB continues the communication (RA procedure) with the eNB, makes settings for the wireless channel with the eNB, and so on. The UE that was not selected by the eNB tries to retry an RA after a predetermined time interval or otherwise.
Note that such a contention of a signature occurs while a UE makes a handover to switch between eNBs for connecting, the connection might be temporarily disconnected or sometimes a communication might be disconnected. For this purpose, the LTE proposes to allocate individual signatures in advance to each of UEs for making a handover. Such an RA procedure is referred to as “non-contention based random access procedure (non-contention based RA procedure)”.    Non-Patent Document 1: 3GPP TR25.913 V7.3.0 (2006-03)    Non-Patent Document 2: 3GPP TS36.300 V8.4.0 (2008-03)
Even when a UE that makes a contention based RA is selected by the eNB as a UE to establish a connection, the UE may not be refused connection by the eNB. For example, the eNB may refuse or suspend a connection with a new UE when the communication traffic is high.
In such a case, although the UE can execute connection processing (RA) to the eNB, such as after a predetermined time interval, the eNB is not aware of which UE was selected in the previous RA procedure. Therefore, conventionally, even if a UE was selected by the eNB in the previous RA procedure, that UE can execute connection processing (RA) in the same priority (procedure) as those of other UEs that were not selected.